Predicting evaporation from mountain streams

Abstract

Evaporation can be an important control on stream temperature, particularly in summer when it acts to limit daily maximum stream temperature. Evaporation from streams is usually modelled with the use of a wind function that includes empirically derived coefficients. A small number of studies derived wind functions for individual streams; the fitted parameters varied substantially among sites. In this study, stream evaporation and above-stream meteorological conditions (at 0.5 and 1.5 m above the water surface) were measured at nine mountain streams in southwestern British Columbia, Canada, covering a range of stream widths, temperatures, and riparian vegetation. Evaporation was measured on 20 site-days in total, at approximately hourly intervals, using nine floating evaporation pans distributed across the channels. The wind function was fit using mixed-effects models to account for among-stream variability in the parameters. The fixed-effects parameters were tested using leave-onesite-out cross-validation. The model based on 0.5 m measurements provided improved model performance compared to that based on 1.5 m values, with RMSE of 0.0162 and 0.0187 mm h −1 , respectively, relative to a mean evaporation rate of 0.06 mm h −1. Inclusion of atmospheric stability and canopy openness as predictors improved model performance when using the 1.5 m meteorological measurements, with minimal improvement when based on 0.5 m measurements. Of the wind functions reported in the literature, two performed reasonably while five others exhibited substantial bias.

Predicting Latent and Sensible Heat Fluxes in Stream Temperature Models: Current Challenges and Potential Solutions

“…Therefore, the Penman coefficients appear to have a reasonable amount of empirical support for application to streams. Szeitz and Moore (2020) found that the Brady et al. (1969) equation performed almost as well as the Penman wind function.…”

“…(1969) wind functions with apparently reasonable results (Garner et al., 2014; King & Neilson, 2019; Leach & Moore, 2011; Magnusson et al., 2012). The Penman coefficients were based on empirical data over a range of “wet” surfaces (Penman, 1948, 1956), and Szeitz and Moore (2020) found that they predicted with an uncertainty of 14 W (±20% of the mean flux) compared to measurements using in‐stream pans at nine sites in southwest British Columbia. Therefore, the Penman coefficients appear to have a reasonable amount of empirical support for application to streams.…”

“…Mass transfer functions for streams have been derived by energy‐balance analysis for artificial streams or canals in three studies (Fulford & Sturm, 1984; Gulliver & Stefan, 1986; Jobson, 1980) and by using in‐stream evaporation pans in five studies of natural streams (Benner, 2000; Caissie, 2016; Guenther et al., 2012; Maheu et al., 2014; Szeitz & Moore, 2020). Even just considering the most comparable studies (Caissie, 2016; Guenther et al., 2012; Maheu et al., 2014; Szeitz & Moore, 2020), the fitted wind functions vary substantially. For example, the coefficient ranges over half an order of magnitude (Table 1).…”

“…Studies by Guenther et al. (2012) and Szeitz and Moore (2020) were both conducted in southwest British Columbia.…”

“…"CB" indicates Catamaran Brook and "LWSM" indicates Little Southwest Miramichi River, both in New Brunswick. Studies by Guenther et al (2012) and Szeitz and Moore (2020) were both conducted in southwest British Columbia.…”